Can't do validation because mule doesn't see payload - mule

Question

I'm trying to use validation connector to check value field for outcome "good" or "bad". Even if they are written in capital letters. The issue is when I use the validation connector to check payload.value it shows null. This is not the case when I debug on payload that shows the full payload. How can I solve it?
I have my xml code below:
<?xml version="1.0" encoding="UTF-8"?>
<mule xmlns:dw="http://www.mulesoft.org/schema/mule/ee/dw" xmlns:http="http://www.mulesoft.org/schema/mule/http" xmlns:validation="http://www.mulesoft.org/schema/mule/validation" xmlns="http://www.mulesoft.org/schema/mule/core" xmlns:doc="http://www.mulesoft.org/schema/mule/documentation"
xmlns:spring="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-current.xsd
http://www.mulesoft.org/schema/mule/core http://www.mulesoft.org/schema/mule/core/current/mule.xsd
http://www.mulesoft.org/schema/mule/http http://www.mulesoft.org/schema/mule/http/current/mule-http.xsd
http://www.mulesoft.org/schema/mule/validation http://www.mulesoft.org/schema/mule/validation/current/mule-validation.xsd
http://www.mulesoft.org/schema/mule/ee/dw http://www.mulesoft.org/schema/mule/ee/dw/current/dw.xsd">
<http:listener-config name="HTTP_Listener_Configuration" host="0.0.0.0" port="8081" doc:name="HTTP Listener Configuration"/>
<flow name="testFlow">
<http:listener config-ref="HTTP_Listener_Configuration" path="/test" doc:name="HTTP"/>
<dw:transform-message doc:name="Transform Message">
<dw:set-payload><![CDATA[%dw 1.0
%output application/xml
---
{Service: {
Id: "N2K99",
Value: "good"
}
}]]></dw:set-payload>
</dw:transform-message>
<validation:matches-regex value="#[payload.Service.Value]" regex="^(good|bad)" doc:name="Validate good or bad string"/>
<dw:transform-message doc:name="Transform Message">
<dw:set-payload><![CDATA[%dw 1.0
%output application/json
---
{
succes: hallo
}]]></dw:set-payload>
</dw:transform-message>
</flow>
</mule>
Error:
Message : Execution of the expression "payload.Service.Value" failed. (org.mule.api.expression.ExpressionRuntimeException).
Payload : <?xml version='1.0' encoding='UTF-8'?>
<Service>
<Id>N2K99</Id>
<Value>good</Value>
</Service>
Element : /testFlow/processors/1 # so-validate-regex
--------------------------------------------------------------------------------
Root Exception stack trace:
java.lang.NullPointerException
at org.mule.mvel2.optimizers.impl.refl.ReflectiveAccessorOptimizer.compileGetChain(ReflectiveAccessorOptimizer.java:440)
at org.mule.mvel2.optimizers.impl.refl.ReflectiveAccessorOptimizer.optimizeAccessor(ReflectiveAccessorOptimizer.java:212)
at org.mule.mvel2.ast.ASTNode.optimize(ASTNode.java:160)
at org.mule.mvel2.ast.ASTNode.getReducedValueAccelerated(ASTNode.java:116)
at org.mule.mvel2.MVELRuntime.execute(MVELRuntime.java:86)
at org.mule.mvel2.compiler.CompiledExpression.getDirectValue(CompiledExpression.java:123)
at org.mule.mvel2.compiler.CompiledExpression.getValue(CompiledExpression.java:119)
at org.mule.mvel2.compiler.CompiledExpression.getValue(CompiledExpression.java:113)
at org.mule.mvel2.MVEL.executeExpression(MVEL.java:953)
at org.mule.el.mvel.MVELExpressionExecutor.execute(MVELExpressionExecutor.java:87)
at org.mule.el.mvel.MVELExpressionLanguage.evaluateInternal(MVELExpressionLanguage.java:228)
at org.mule.el.mvel.MVELExpressionLanguage.evaluate(MVELExpressionLanguage.java:163)
at org.mule.el.mvel.MVELExpressionLanguage.evaluate(MVELExpressionLanguage.java:142)
at org.mule.expression.DefaultExpressionManager.evaluate(DefaultExpressionManager.java:217)
at org.mule.expression.DefaultExpressionManager.evaluate(DefaultExpressionManager.java:188)
at org.mule.util.AttributeEvaluator.resolveValue(AttributeEvaluator.java:124)
at org.mule.module.extension.internal.runtime.resolver.TypeSafeExpressionValueResolver$PassThroughEvaluatorDelegate.resolveValue(TypeSafeExpressionValueResolver.java:147)
at org.mule.module.extension.internal.runtime.resolver.TypeSafeExpressionValueResolver$CaptureContextEvaluatorDelegate.resolveValue(TypeSafeExpressionValueResolver.java:137)
...

Answer 1

The problem that you are having is because the first DataWeave transformation -which I understand is used to mock the input- returns an XML string. This is Mule 3 so the validation uses a MEL expression to try to evaluate its input #[payload.Service.Value]. MEL doesn't know how to parse the XML so it fails with an error.
To resolve the issue you need only to transform the input into a Java object that MEL can evaluate. For example putting this transformation between the first Transform and the validate resolves the issue:
<dw:transform-message doc:name="Transform Message">
<dw:set-payload><![CDATA[%dw 1.0
%output application/java
---
payload
]]>
</dw:set-payload>
</dw:transform-message>